The study involves 20 visits over a 6-month period. An exercise-training program will be designed for the individual's fitness level. During the first month, the individual will be supervised while exercising at Spaulding Rehabilitation Hospital, but after that may choose to exercise at home or in a fitness center. These exercise sessions will take place 3-5 times per week for 15-40 minutes per session based on the individual's exercise program, increasing to 4-6 times per week for 30-60 minutes per session after the first month. Participants will be required to visit Spaulding Rehabilitation Hospital a minimum of once every two weeks so that information can be collected from a heart rate monitor.